Hi everyone welcome back to Cody's lab.
So today I want to try making a model of a confined aquifer.
So what we've got here is the basin which is going to be holding the water and actually for the well, I'm gonna be using a
bubbler stone connected to this clear tubing here
So you basically got the hole in the ground - which is the well
- and then down near the bottom where the aquifer is you got the filter pack. Ya know.
This is to keep the sand from going into the hole. Now I've got some water that's dyed blue
so it'd make it easier to see. I've got some clay and
Down by my feet are some buckets full of sand.
So the sand is gonna be our water storage medium in the actual aquifer.
Let's start putting in the sand here
So think of this like a river is currently depositing sand here
Or gravel or something that is good at transferring water
There's our transmissive layer of sand
Which can hold water. Now of course this probably in real life would be already full of water
So let's start adding the water here
And now what we've got is an unconfined aquifer basically the water is only pressurized to
you know atmospheric pressure and its own head height so with this type of aquifer all we have to do is dig a hole and
The hole will fill up with water as the water comes in from the sand
You know maybe you've got your river channel
You know the water flows into the river channel because it's a lower spot
If I dig the sand away, you should be able to see that the water
Level inside of our well bore here is about the same as the water level of
the aquifer see if I pull water away you can see
It'll begin to drop and then the water comes back up
It's a little bit higher, and that's due to capillary action, okay, so let's push the sand back up against this
Level this all out
And now we need to add a layer of something that slows down the flow of water
An aquitard. If I'm remembering my terminology correctly.
Now clay is the obvious answer for what this aquitard will be made out of because clay
Absorbs water and it makes it very difficult for the water to go through it. It forms an almost impenetrable layer
now how you would get this is say the river meandered away and
Then you start depositing smaller material or perhaps a lake formed
And you get some sediment at the bottom of the lake
But ultimately this forms a cap that the water has difficulty moving through
this will confine that lower aquifer there
Hence the name of confined aquifer
Okay, now that I've got my clay layer in
I'm gonna have to let this absorb water
Which is going to be slow because water moves through clay slowly. I might even have to spritz in some from the top here
I'll give it an hour or two to get fully moistened
So there's the clay in and that's thoroughly hydrated.
I actually ended up adding a layer of bentonite you can see here
just to make sure it seals very well, and you can already see a
little something here
See that, the water level has been pushed up
Above the layer of clay
That's because the weight of the clay is now pushing down and since it's heavier than water would be,
it's creating more ... pressure. so now the pressure and this
Aquifer is now high enough to actually force the water all the way out to the ground surface
But the clay is holding it down so the water can't just go right up through the clay
Let's turn this up to 11 and add another
Aquifer on top up here. Let's use a different colored sand shall we?
I almost forgot, but let's add a second well in here into the unconfined aquifer that we're making
this way we can compare the water height
oh no, we got some water escaping
Look at that the water has found its way up through clay
That's annoying
It's gonna ruin my test
Is there a way I can plug this?
Yeah, but
Is there gonna be others like that?
So I just filled all the empty spaces in the sand with water. In fact, I've now got standing water up here to the top
So I guess this would be
let's call this a river channel right here a really wide river or something
so the unconfined aquifer is just about even with the
water table
And the confined aquifer, has got the water pushed all the way up here
Essentially what I've got here is an artesian spring or an artesian well. see?
The water is naturally coming up out of the ground under pressure
And I'll be able to get a little bit of water down here
You might be asking. "Where is this water coming from?" You see, water can't flow down; like you can't go from here
passed the clay layer and then to here the water is coming from what was trapped in here
the sand is
compacting under the load
But the water is actually forming just a little bit of the back pressure keeping the sand from compacting
but as the water leaves the sand can compact and so the actual entire ground surface is dropping slightly as
The water is being pumped out
...or pumping itself out
By the weight of the ground above it
it's able to do this because the sand has a higher density than water and
you've got water in it but...
The sand itself is so heavy
It's able to overcome the hydraulic gradient and force the water higher than you would normally expect
And that's actually pumping out quite a lot of water
it makes sense you can store a lot of water this way, but
These are very difficult to water back.
These confined aquifers do recharge, but very slowly
And usually not straight down or rather
Water that's coming out from maybe some mountains
It just goes really deep and then tries to come back up.
Once this ground is compacted. It's not gonna just spring back naturally.
Once this is compacted, It's
It's lost your "store-ativity" forever.
In many places of the world such as California
This is a huge problem because those aquifers - which are being used because there's a drought and they need the water...
Are never gonna recover and the ground sinking can cause other problems as well.
There's a little bit of storage you can get at that point due to the compressibility of water itself, but it's not very much.
In fact, I think this is just about done
Yeah
So it's compacted you could add more weight here
And I could force more water out by compacting the sand tighter.
Gonna push on the sand ... Increase the pressure
... then it'll pop out
Increasing the pressure does cause more water to force out so this sand isn't fully compacted just yet
That's not really a practical thing to do
Now for this unconfined aquifer. I actually have to do work to pump the water out.
You might notice that
The water here is a different color because I didn't add the dye to it
And also it's a lot more murky murky
It's not a good water quality
But that's mostly just because of the type of sand I've got it with watch
What happens when I've got some wells spaced out away from the pumping well
Maybe call these ponds, but let's just start pumping. I'm just gonna suck the water out with my mouth
See the first one has already been emptied because it's right next to the well
See that? We should have a cone-shaped depression in the surface of the water
What we've caused is a cone-of-depression. This whole area here is now
without water
the water has got to flow in from elsewhere and
There might actually not be enough water to do that, so let's just add the water back a little bit here
We should start seeing the water come back. Yeah, so in this case it came
Kind of off to the side here
You can see that? that's because the water has
Trouble flowing through the sand it actually creates a resistance that slows this water's flow down
This means that if it's close to the well the water can flow
to it much quicker
Also, the farther you get from the well the more
volume of water you have within that radius and
that's how these confined aquifers can actually hold on to water even if the
Clay layer above them doesn't completely cap it off
That's because the water here has to move through the material to find a way out
That's why I had a little bit of time to work
To actually pump a little bit of water out of it
Because the water couldn't just flow out instantaneously.
A confined aquifer would also form a sort of cone of depression, but it's not actually the water's level
But rather the potential head - a potentiometric surface - that gets drawn into a cone.
So basically you have lower pressure near the well and higher pressure elsewhere
I'm gonna try this again that is kind of fun watching the cone of depression expand out
Here, let's not pull out as much water out, so now I should be able to see them recover and stop pumping here
Yeah there we go
Alright
This's a lot of fun for me
Not sure if I conveyed the information nearly as well as my geology teacher. But, I hope you enjoyed at least!
and I'll see you next time
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